K.,
I am going through a similar situation right now. My daughter is 2.5 (will be three in January), and last Thursday, out of nowhere, she started waking at night, screaming out for me. She has been sleeping through the night since she was 9-months-old--no problems--she didn't even have a problem when we switched her room due to a new sibling arriving and putting her into a twin bed. I am beside myself these last few nights, not to mention tired because I also have my 12-week-old who is waking to eat in the early AM.
I have a feeling she is scared about something, but she has a speech delay and is unable to communicate to me if she is scared/what she is scared about. Luckily, she does not get out of her bed without me coming into her room. I don't know why, but I'm not going to try to change that. Still, she just wakes up and screams for me until I come. I started getting mad about it, but that only made it worse. I don't want to leave her if she's truly scared, but I can't get up 2-3 times a night and sit in her room with her.
I saw some things online about role playing during the day so that you can get a better idea of why she is doing it. Use a doll and set up a night-time situation where she can pretend with the doll. Maybe if you do that she will kind of let you in on if she's scared (but not admitting it), or if she thinks it's funny or a joke.
For us, we are still trying to get to the root of the problem...I hope something works soon!
Good Luck to you!